The tidal characteristics of the English Channel are the controlling factor for tides in the Solent. Low water at Penzance occurs at approximately the same time as high water in the eastern reaches. This indicates that there is a degree of resonance within the channel; the natural period of the channel being approximately 10 hours (Webber, 1980).
The structure of the tides in the Channel, and the resonance in particular, produces rapid increases in the tidal range close to the Isle of Wight. This is very apparent from the Nab Tower to Christchurch Bay (where the tidal range doubles in a distance of approximately 80 km) and also within the Western Solent where major changes occur over about 16 kilometres. To the west of the Solent (inland of Weymouth) is an amphidrome for the main tidal constituent. Close to the amphidromic point, this semi-diurnal harmonic constituent is relatively weak. However, the configuration of Christchurch Bay produces strong shallow water effects, which interact with the phase of the main tidal harmonic to generate the double high water. Further east, along Spithead in the eastern Solent, the phase relationship alters slightly, resulting in an extended rather than a double high water.
In Southampton Water the tidal characteristics are unique, and are described by a “young flood stand” and a double high water where there is little change in water level (lasting for up to 3 hours). The stand on the flood is most pronounced on spring tides and can last for about 2 hours. The tidal profiles are also asymmetric with the ebb phase of the tide taking about 5 hours compared to over 7 hours for the flood.
The double tide in the Solent (and across the channel to the French coast) not only resonates itself along the English Channel, but also has a contributory factor from the tide that has gone up the West Coast, across the top of Scotland, and then come down the North Sea from the North.
The major North Atlantic amphidromic point is about two thirds of the way towards America going west from Cornwall. The semidiurnal tidal wave from the tip of South Africa comes north up the west coast of Africa and joins with the eastern side of the North Atlantic circulation. It passes the tip of west Africa and two hours later it reaches the south of Spain and an hour after that passes the north of Spain. About an hour after that it first reaches St Mary’s and Newlyn on the South west tip of Britain. It has been building in height and at the Uk is some three times higher than when it first reached the tip of west Africa.
The ‘wedge’ of Cornwall divides the wave. It continues east along the south coast. However in the Northern hemisphere waves tend to the right [Coriolis effect] resulting in much lower tides on the English coast and much higher on the French coast. In the centre of the South coast is a minor amphidromic point where the tide ‘breaks away’ from the coast, resulting in the much lower range ‘double tide’ section from Weymouth to Portsmouth. This section marks the furthest point influenced also by the tide which has come down the North Sea which is the Atlantic tide (as at St Mary’s) delayed by about two and a half wavelengths. The South coast tide is about a half wavelength from west to east and therefore at the meeting point more or less at Dover the two tides which have come by two different tidal routes are roughly in phase. The area of meeting of the two tides stretches from the Isle of Wight right round to the coast of Suffolk where its last influence is seen at Lowestoft.
The tide along the North Cornwall Coast, keeping right, is much higher and it swings into the Severn Estuary, building as it goes until it becomes the highest tide of British shores and almost in the world. It sometimes results in the Severn bore.
As it runs into the Severn estuary it also comes east along the South coast of Wales. Note therefore that the tides of Great Britain all run clockwise with the two exceptions of the South Coast from St Mary’s to Dover and the South Coast of Wales from Milford Haven to Avonmouth.
The tide runs north into the Irish Sea, past the west coast of Wales, swinging right to give a high range of tide at Liverpool. The tides on the Irish coast of the North Sea are much lower in height. At the exit of the Irish sea to the north the tide is effectively blocked not just by the narrowness of the channel but also because it is well out of phase with the Atlantic tidal wave coming north of Ireland. It will be seen the the tide at Port Ellen is very small and frequently unstable.
The Atlantic tide reaches Tobermory on the west coast of Scotland. It sweeps around the north of Scotland and breaking away from the Atlantic circulation, keeping right turns south and comes down the east coast. By the time it reaches Dover it meets the direct tide from the south coast. The wave form at Dover sometimes shows the ‘stress’ that is caused by the merging of these two waves with differing phases. The time from Low to High Water is at times only half what it is from High water to Low water.
The amphidromic points significant to Great Britain are therefore:
The major North Atlantic circulation;
The minor amphidromic point near the New Forest in the centre of the South Coast of England;
The minor amphidromic point in the narrow channel between Ireland and Scotland;
The amphidromic point on the south western coast of Norway around which the tides of the northern part of the North Sea revolve;
The amphidromic point in the centre of the North Sea;
And the amphidromic point in the Southern North Sea.
In the Southern North Sea the tide coming down the east Coast meets the tide coming past Dover which helps to ‘turn it’ across the channel so that it proceeds north up the Belgian and Dutch Coasts before swinging across towards Cromer to reinforce the next tide coming down the east coast.
And so, in summary, we have a slosh due to the harmonic frequency of the English Channel, the Coriolis Effect tending the tide to turn right in the northern hemisphere, the harmonic from the tide which travelled around Scotland and down the East Coast, the shallow nature of Christchurch Bay, and the deprecated amphidromic point near Weymouth which gives such entertaining tidal patterns in and around the Solent.
Tides in Poole
The tide in Poole Harbour and around Poole has a very special profile – very different from the rule of 12ths, and of course it’s complex. Poole lies about half way between one end of the English Channel and the other, and it’s close to a minor deprecated amphidromic point. Deprecated means that it’s a point on the planet where the tide height would not go up and down if it were sea, but it’s actually land.
Think of the English Channel as a bath half full of water. If the water is still, and then you dip your hands into it and begin an oscillation at the same frequency as the bath, the water would naturally slosh each way reflecting off the ends of the bath. The water in the exact middle of the bath would stay the same height, although the water itself would pass left and right with each oscillation. Since the English Channel has a natural frequency of nearly the frequency of the incoming tide, the oscillation is nudged by the tide on every cycle.
Poole is near the deprecated amphidromic point, which is somewhere near Dorchester, a few miles inland of Poole. The water in Poole wants to stay mostly the same height, but every tide it ebbs quickly and floods quickly back to it’s “stable” height.
Looking at the profile of height from neaps to springs, it can be seen that there is a central height to the tide as Poole, which then fluctuates dramatically with a strong ebb and flood back to the central height, then it floods and ebbs back to the central height. See the wikipedia entry for Amphidromic points for more details of worldwide amphidromes.